1. Field of the Invention
The present invention relates in general to a hybrid type resinoid grindstone which has a resinoid abrasive structure containing abrasive aggregates or agglomerates each having a vitrified abrasive structure, and more particularly to such a hybrid type resinoid grindstone in which each of the abrasive agglomerates includes sol-gel abrasive grains held together by a vitrified bonding agent.
2. Discussion of the Related Art
There is known a resinoid grindstone in which abrasive grains are held together by a thermosetting synthetic resin used as a bonding agent. The resinoid grindstone is elastically deformed in a larger amount when a grinding load is applied thereto, than a vitrified grindstone. Further, the resinoid grindstone has a smaller degree of strength for holding or bonding the abrasive grains together, and a lower degree of wear resistance than the vitrified grindstone. Due to such characteristics thereof, the resinoid grindstone is not capable of satisfying a recent demand for grinding or removing a larger volume of material from a workpiece with a higher efficiency, even where the abrasive structure of the resinoid grindstone is modified to have a higher bonding strength and a higher density.
Where the grindstone takes the form of a gear grinding wheel having a tooth profile which is formed in its outer or inner circumferential surface and which conforms to a tooth profile of a gear to be honed or ground as a workpiece, the grinding wheel has to be periodically or cyclically dressed so as to maintain its tooth profile so that the tooth profile of the gear formed by the grinding wheel is so maintained. That is, when the tooth profile is changed to a certain extent due to wear of the grinding wheel, or when the amount of wear of the grinding wheel exceeds a predetermined amount, the grinding wheel has to be subjected to a dressing operation so as to rectify or reshape the tooth profile. The dressing cycle or interval is closely related with a service life of the grinding wheel. The wear resistance of the grinding wheel could be improved by using a resinoid grindstone of hard grade or a resinoid grindstone having a high bonding strength. However, such a resinoid grindstone of hard grade does not have a high degree of sharpness and is not accordingly capable of removing a large volume of material from a workpiece with a high efficiency.
In an attempt to increase the rate of material removal from a workpiece with an increased degree of sharpness, there is proposed a resinoid grindstone, as disclosed in EP 0692342 A2, in which sintered alumina abrasive grains are held together by a vitrified bonding agent so as to form each of abrasive agglomerates, and the abrasive agglomerates are held together by an epoxy or other thermosetting synthetic resin. This resinoid grindstone exhibits an increased degree of bonding strength for bonding the abrasive grains, but suffers from breakage or wear of each abrasive grain in a relatively short time after the initial use. Thus, the proposed resinoid grindstone is not satisfactory in order to prolong the dressing interval and the service life of the grindstone.
There is also proposed a resinoid grindstone in which so-called sol-gel abrasive grains are held together by a bonding agent in the form of a thermosetting synthetic resin. The sol-gel abrasive grains are polycrystals each of which is obtained in a sol-gel method by combining submicron-sized, fine crystals together with each other, so that each of the sol-gel abrasive grains breaks down to expose a new sharp cutting edge as each sol-gel abrasive grain becomes dull. That is, the sol-gel abrasive grain is expected to present a new sharp cutting edge as a result of small breakage of the abrasive grain, which breakage is smaller than that in an ordinary sintered abrasive grain. In other words, the sol-gel abrasive grain is expected to restore its sharpness as a result of each of the successive removals of the fine crystals. However, such expected restoration of the sharpness is not satisfactorily achieved in this proposed resinoid grindstone. This is probably because an impact or shock, which is required to be applied to the abrasive grain for causing the small breakage of the sol-gel abrasive grain or the removal of the fine abrasive grain, is actually absorbed by elasticity of the thermosetting synthetic resin which holds the sol-gel abrasive grains, so that the expected restoration of the sharpness is not satisfactorily achieved in this proposed resinoid grindstone.
It is therefore an object of the present invention to provide a resinoid grindstone which has a high degree of sharpness while having an increased degree of wear resistance with a high degree of strength of bonding abrasive grains, and which is accordingly capable of grinding a workpiece to remove a large amount of stock with a high efficiency.
This object may be achieved according to the principle of the present invention, which provides a hybrid type resinoid grindstone comprising abrasive agglomerates which are held together by a thermosetting synthetic resin used as a bonding agent, wherein each of the abrasive agglomerates includes sol-gel abrasive grains which are held together by a vitrified bonding agent.
In the hybrid type resinoid grindstone of the present invention, the abrasive agglomerates, each of which is formed of the plurality of sol-gel abrasive grains and is accordingly lager than each sol-gel abrasive grain, are firmly bonded to each other by the bonding agent in the form of the thermosetting synthetic resin. Each of the sol-gel abrasive grains, which are held together by the vitrified bonding agent, restores its sharpness as a result of each of the successive removals of fine crystals with application of an impact or shock to the sol-gel abrasive grain, as in a vitrified grindstone. Thus, the present hybrid type resinoid grindstone exhibits a high degree of sharpness while having an increased degree of wear resistance with a high degree of bonding strength, and is accordingly capable of grinding a workpiece to remove a large amount of stock from the workpiece with a high efficiency.
According to a first preferred form of the invention, the hybrid type resinoid grindstone contains at least 5%, more preferably at least 20% of the abrasive agglomerates per total weight of all abrasive grains contained in the hybrid type resinoid, for further increasing the degree of sharpness of the hybrid type resinoid grindstone so that the grindstone is capable of further efficiently grinding a workpiece having a large amount of stock.
According to a second preferred form of the hybrid type resinoid grindstone of the invention, each of the sol-gel abrasive grains has a grain size of F60-F100, i.e., about {fraction (1/100)} inch {fraction (1/60)} inch.
According to a third preferred form of the invention, the hybrid resinoid grindstone is a gear grinding wheel including a tooth profile portion having teeth formed therein, and further comprises individual abrasive grains which are held together by the thermosetting synthetic resin.
The hybrid type resinoid grindstone of this third preferred from of the invention is the gear grinding wheel in which the individual abrasive grains, as well as the abrasive agglomerates, are held together by an epoxy resin or other thermosetting synthetic resin. Each of the sol-gel abrasive grains which are held together by the vitrified bonding agent restores its sharpness as a result of the successive removals of the fine crystals upon application of an impact or shock to the sol-gel abrasive grain, as in a vitrified grindstone, during a grinding operation, even if the present gear grinding wheel is adapted to have an increased degree of bonding strength and an accordingly increased degree of wear resistance so as to hone or grind the tooth faces of a gear which has been hardened in its surface. Thus, the present gear grinding wheel is capable of performing a grinding operation in which a large volume of material has to be removed from a workpiece with high accuracy and efficiency.
According to one arrangement of the third preferred form of the hybrid type resinoid grindstone of the invention, the gear grinding wheel has the tooth profile portion in an inner circumferential surface thereof.
According to a fourth preferred form of the invention, the hybrid type resinoid grindstone is a porous epoxy grindstone which is used for grinding a cutter, and further comprises individual abrasive grains which are held together by the thermosetting synthetic resin.
The hybrid type resinoid grindstone of this fourth preferred from of the invention is the porous epoxy grindstone. This porous epoxy grindstone may be produced by a method including a step of mixing the abrasive agglomerates, the individual abrasive grains and the thermosetting synthetic resin in the form of a liquid epoxy resin, so as to form a mixture thereof, and a step of introducing the mixture into a mold and then hardening the liquid epoxy resin. Each of the sol-gel abrasive grains which are held together by the vitrified bonding agent restores its sharpness as a result of the successive removals of the fine crystals upon application of an impact or shock to the sol-gel abrasive grain, as in a vitrified grindstone, during a grinding operation, even if the present porous epoxy grindstone is adapted to have an increased degree of bonding strength and an accordingly increased degree of wear resistance so as to grind a cutter which has been hardened in its surface. Thus, the present porous epoxy grindstone is capable of performing a grinding operation in which a large amount of stock has to be removed from a workpiece with high accuracy and efficiency.